Various kinds of bearing units for wheel support, which are constructed such that an outer race and inner race rotate freely by way of rolling elements, have been used in order to support wheels such that they can rotate freely with respect to the suspension.
Moreover, a bearing unit for supporting the drive wheels on the independent suspension, as well as for rotating and driving the drive wheels, is constructed by combining a bearing unit for supporting the wheels with a constant-velocity joint. This bearing unit for wheel drive must be able to transmit the rotation of the drive shaft to the wheels smoothly (securing constant velocity) regardless of the relative displacement between the differential gear and driven wheels and the steering angle applied to the wheels.
FIG. 1 shows a typical bearing unit for wheel drive for this purpose that comprises a bearing unit 1 for wheel support and a constant-velocity joint 2.
In the bearing unit 1 for wheel support, a hub 4 and inner race 5 are supported on the radial inside of an outer race 3 by way of a plurality of rolling elements 6 such that they rotate freely. Of these, the outer race 3 is fastened to the knuckle (not shown in the figure) of the suspension by a first flange 7, which is formed around the outer peripheral surface of the outer race 3, such that it does not rotate even during use. In addition, a pair of outer-ring raceways 8 are formed around the inner peripheral surface of the outer race 3, the hub 4 and inner race 5 are supported on the radial inside of this outer race 3 such that they are concentric with the outer race 3 and such that they rotate freely.
A second flange 9 for supporting the wheel is formed around the outer peripheral surface of the hub 4 at a portion near the outside end thereof (the outside is the side which is on the outside in the width direction of the vehicle when the bearing unit is installed in the vehicle, and in the figures that show the bearing unit for wheel support, including FIG. 1, it is on the left side). Also, a first inner-ring raceway 10 is formed around the outer peripheral surface in the middle of the hub 4, and a small-diameter stepped section 11 is formed around the outer peripheral surface on the inside end of the hub 4 (the inside is the side which is on the center in the width direction of the vehicle when the bearing unit is installed in the vehicle, and in the figures that show the bearing unit for wheel support, including FIG. 1, it is on the right side), and the inner race 5 is fitted around the small-diameter stepped section 11 and fastened to this small-diameter stepped section 11 and a second inner-ring raceway 12 is formed around the outer peripheral surface of the inner race 5. In addition, a first spline hole 13 is formed in the center of the hub 4.
On the other hand, the constant-velocity joint 2 comprises an outer ring 14 for the constant-velocity joint, an inner ring 15 for the constant-velocity joint, a plurality of balls 16 and a spline shaft 17. The outer ring 14 for the constant-velocity joint and the spline shaft 17 form a drive-shaft member 18. In other words, this spline shaft 17 is formed on the outer half of the drive-shaft member 18, and it is fitted in the spline hole 13, while the outer ring 14 for the constant-velocity joint is formed on the inner half of the drive-shaft member 18. Outside engagement grooves 19 are formed at a plurality of locations in the circumferential direction around the inner peripheral surface of the outer ring 14 for the constant-velocity joint, such that the outside engagement grooves 19 are orthogonal to this circumferential direction.
Moreover, a second spline hole 20 is formed in the center of the inner ring 15 for the constant-velocity joint, and the inside engagement grooves 21 are formed around the outer peripheral surface of the inner ring 15 in alignment with the outside engagement grooves 19, and orthogonal with respect to the circumferential direction.
The aforementioned balls 16 are held between the inside engagement grooves 21 and the outside engagement grooves 19 by a retainer 22 such that they roll freely along the engagement grooves 21, 19.
The shape of the components of the constant-velocity joint 2 are the same as those for the well known Rzeppa-type or Barfield-type constant-velocity joint, and are not related to this invention so a detailed explanation of them is omitted.
For the constant-velocity joint 2 described above and this kind of rolling bearing unit 1 for wheel support, the spline shaft 17 is inserted into the spline hole 13 from the inside toward the outside. Also, on the outside end of the spline shaft 17, a nut 24 screws onto a male screw section 23 that is formed on the part that protrudes from the outside end of the hub 4, and by tightening this nut 24, the spline shaft 17 and hub 4 are fastened together. In this state, the inside end surface of the inner race 5 comes in contact with the outside end surface of the outer ring 14 for the constant-velocity joint, so the inner race 5 does not move in a direction such that it would come apart from the small-diameter stepped section 11. At the same time, adequate pre-stressing is applied to the rolling elements 6.
Furthermore, when assembled with the suspension of the vehicle, the male spline section 26 that is formed on the outside end of the drive shaft 25 is fitted in the second spline hole 20 that is formed in the center of the inner ring 15 for the constant-velocity joint.
Also, the snap ring 28 that is fitted in the attachment groove 27 that is formed all the way around the outer peripheral surface on the outside end of the male spline 26 is engaged with the stepped section 29 that is formed around the edge of the opening on the outside end of the second spline hole 20 to prevent the male spline section 26 from coming out from the second spline hole 20. The inside end of the drive shaft 25 is fastened to the center of the trunnion of a tripod-type constant-velocity joint (not shown in the figure) that is formed on the output shaft of the differential gear (not shown in the figure). Accordingly, the drive shaft 25 rotates at constant velocity when the vehicle is running.
In the case of the bearing unit for wheel drive shown in FIG. 1, the force that presses the inner race 5 so as to apply a pre-load to the rolling elements 6 that are located between the first and second inner-ring raceways 10, 12 and each of the outer-ring raceways 8, is obtained by screwing the nut 24 onto the male screw section 23 and tightening it. Therefore, it is necessary to tighten the nut 24 very tightly to secure the pressure force of the inner race 5.
The size of the shaft force that occurs in the spline shaft 17 and that presses the outside end surface of the outer ring 14 of the constant-velocity joint against the inside end surface of the inner race 5 by tightening the nut 24 on the male screw section 23 is substantially large and depends on the size of the bearing unit for wheel drive, such that it is about (4 to 9)×104 N for a normal passenger car.
Due to this large shaft force, surface pressure is applied to the area of contact between the inside end surface of the inner race 5 and the outside end surface of the outer ring 14 of the constant-velocity joint, however, both of these end surfaces are a planar surface that exists in a direction perpendicular to the center axis, and therefore come in contact over a wide area with each other. Therefore, neither of the end surfaces have plastic deformation at the area of contact.
Moreover, with the construction disclosed in Japanese Patent Publication No. Tokukai Hei 11-5404, and as shown in FIG. 2, the cylindrical section existing on the inside end of the hub 4 at a portion that protrudes inward beyond the inner race 5 that is fitted over the small-diameter stepped section 11 forms a crimped section 30 that is deformed outward in the radial direction, and the inner race 5 is retained toward the stepped surface 31 of the small-diameter stepped section 11 by this crimped section 30. In the case of this second example of prior construction, a pre-load is applied to the rolling elements 6 by the retaining force of the crimped section 30. Similar to the first example of prior construction described above, the connection between the bearing unit 1 for supporting the wheels and the constant-velocity joint 2 is obtained by screwing the nut 24 onto the male screw section 23 that is formed on the outside end of the spline shaft 17 and tightening it. When this nut 24 is tightened, the flat surface 32 that is formed on the inner surface of the crimped section 30 comes in contact with the surface on the outside end of the outer ring 14 of the constant-velocity joint. In the case of this second example of prior construction as well, the aforementioned flat surface 32 is formed such that shaft force that occurs in the spline shaft 17 when the nut 24 is screwed onto the male screw section 23 and tightened can become large as in the case of the first example of prior construction.
With the bearing unit for wheel drive as shown in FIG. 1 and as described above, an unpleasant rubbing noise of squeal sometimes occurred when the vehicles was moving. The occurrence of this unpleasant noise is known to be caused when the area of contact between the surface on the inside end of the inner race 5 and the surface on the outside end of the outer ring 14 of the constant-velocity joint rub due to fluctuations in torque that is transmitted between the constant-velocity joint 2 and the bearing unit 1 for supporting the wheel. In other words, the torque changes frequently due to repeated acceleration and deceleration. Moreover, the spline shaft 17 that is formed on the side of the constant-velocity joint 2 elastically deforms in the twisting direction as torque is transmitted between the constant-velocity joint 2 and bearing unit 1 for wheel support, and the amount of deformation tends to change frequently as the torque fluctuates.
Also, as the force, which causes the spline shaft 17 to deform in the twisting direction, or the force, which tries to return the twisted spline shaft 17 to the original position, becomes larger than friction acting on the area of contact, minute slipping occurs at this area of contact. In this case, as the friction force acting on the area of contact becomes large, the rubbing energy between the surface on the inside end of the inner race 5 and the surface on the outside end of the outer ring 14 of the constant-velocity joint becomes large due to the slipping, and cause an unpleasant noise.
In order to prevent the occurrence of this kind of unpleasant noise, a film for reducing the friction was formed using grease, molybdenum disulfide, fluorine resin, or the like, on the area of contact between the surface on the inside end of the inner race 5 and the surface on the outside end of the outer ring 14 of the constant-velocity joint. By making the area of contact slippery, it is possible to keep the rubbing energy between the surface on the inside end of the inner race 5 and the surface on the outside end of the outer ring 14 of the constant-velocity joint small even when minute slipping occurs, and thus it is possible to make it difficult for the unpleasant noise to occur.
It is known that this kind of method is somewhat effective. However, the film for reducing the friction is not always durable enough, and it is difficult to maintain sufficient effect over a long period of time. Particularly, with construction of not sealing the contact area with a seal ring, the period of time that the unpleasant noise can be effectively reduced is limited.
It is also thought that in order to reduce the friction, the tightening force on the nut 24 can be loosened to reduce the surface pressure between the surface on the inside end of the inner race 5 and the surface on the outside end of the outer ring 14 of the constant-velocity joint. However, in the case of the first example of prior art construction shown in FIG. 1, a pre-load is applied to the rolling elements 6 due to the tightening force of the nut 24, so application of this method is difficult. In the case of the second example shown in FIG. 2 as well, increasing the tightening force of the nut 24 as in the case of the first example is considered, so it is not possible to suppress the occurrence of the unpleasant noise.
Furthermore, in the case of the first example of prior art construction shown in FIG. 1, the tightening force of the nut 24 with respect to the male screw section 23 is increased in order to apply a pre-load, so a large axial force occurs in the spline shaft 17. Therefore, the torque required for tightening the nut 24 becomes large, and it is not possible to avoid decrease in work efficiency of assembling the bearing unit for wheel drive.
Taking into consideration that the same axial force that occurred in the first example also occurs in the second example of prior art construction shown in FIG. 2, a large flat surface 32 is formed on the surface of the inside end of the crimped section 30, and therefore, the same problem occurs. Moreover, in the case of the second example shown in FIG. 2, the cost increases due to the work of tightening the nut 24 with a large torque.